The present invention relates to a damping device, in particular intended to form a drag-mode damper for a helicopter blade, and to a rotary-wing aircraft rotor system, such as a helicopter rotor, incorporating this device. The invention applies in a general manner to dampers used not only in the aeronautical sector but also in other industrial sectors which involve angular oscillations about an articulation adjacent to a rotor.
In a known manner, the lifting rotors of helicopters have an articulated-type hub which, for each blade, is provided with an articulation, known as a drag hinge, which is situated about an axis perpendicular to the plane of the blade and which provides said blade with a freedom of angular oscillation in the plane of rotation of the rotor. It is sought to provide an elastic return of each blade toward its mean angular position about this articulation so as to be able to adjust the natural frequency of the whole of each blade in drag, it being pointed out that it is necessary to damp these angular drag oscillations of blades in order to minimize the transmission of dynamic drag forces from each blade toward the structure of the helicopter.
The known drag-damping devices which are used between each blade and the rotor are primarily of two types.
As illustrated in appended FIG. 1, a first type of such devices 1 can be distinguished, this type using only one elastomer damper 2 of annular cross section (usually based on a silicone rubber) which is sheared between the respective walls of an external cylindrical frame 3 and of an internal tube 4. A major disadvantage with this solution, which is very simple to implement, lies, on the one hand, in the very high degree of stiffening of the device with respect to the low excitation amplitudes and, on the other hand, in its excessive sensitivity to the outside temperature. Another disadvantage with this first type of damping devices is that they are especially designed for use on lightweight aircraft.
A second type of drag-damping devices, primarily for large aircraft, can be distinguished, this type using complex hydraulic systems usually based on valves, pistons and calibrated passages, and making it possible to maintain an approximately constant damping level over a wide range of excitation amplitudes. Mention may be made, for example, of document EP-B-0 759 128 for the description of such a device, which comprises two annular elastomer dampers which are mounted around an internal tube and which are separated from one another by an annular compensation chamber having a piston communicating with likewise annular working chambers, it being pointed out that this compensation chamber can also be bounded by a diaphragm or a metal bellows. Although these devices are relatively well-performing, they nevertheless have the disadvantage of entailing a certain degree of technical complexity, which additionally requires a lot of maintenance.